Yes, I have. On my 2001 210 wagon, ironically enough. In my case I was traveling overseas for about 3 weeks. The car was parked in my garage, undriven the entire time. That first Monday morning going back into the office, the brake pedal was very soft. The car was driveable, though it certainly wasn't right. A quick fluid flush fixed it right up.

I had the problem only that one time. It did not occur to me to inspect the flexible brake lines, the car was only 7 or 8 years old at the time, and had low miles - 70K or less.